In conjunction with the DOE Project (DE-FC22-93BC14990), the characterization of a portion of the Permian age San Andres formation under Welch Field, Dawson County, Texas was initiated. The objective of the characterization is the construction of a geologic model for the numerical simulation of a C02 flood of the reservoir. A detailed reservoir description is required because the high costs to inject and recycle C02 dictates a very efficient flood design. Identification of reservoir barriers, flow units, and prediction of continuity present the greatest challenge to the description. The productive portion of the formation is mineralogically simple, consisting of dolomite and anhydrite with very minor amounts of chert and illitic clay. Deposition occurred in a very flat shallow shelf setting ranging from supra tidal to subtidal. Depositional textures are variable ranging from fenestral mudstones to oolitic grainstones, with wackestones and packstones dominating. Diagenesis of these shallow shelf deposits has created a highly complex pore system which at times crosses depositional facies boundaries. Rock fluid interaction further complicated the descriptive process by affecting the log responses.

Porosity-permeability cross plots exhibited good correlation for a carbonate reservoir but were too coarse for simulation purposes. Special core analysis tests of capillary pressure and relative permeability provided the key for identifying the intervals of differing wettability and pore structure. A methodology has been developed to identify the varying pore systems and differing wettabilities, using multiple logs from a routine logging suite and special core analysis. The input from multiple log measurements provides a better estimation of the formation absolute permeability than by using a single porosity-permeability relation. These reservoir parameters could then be extrapolated to uncored wells, using the sequence stratigraphy framework, to build the 3-D model of the reservoir.

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